The Prophet ﷺ said, "Do not exchange insults while you are fasting, and if someone insults you, say, 'I am fasting.'"
حدثنا يحيى بن سعيد عن ابن أبي ذئب قال حدثني عجلان مولى المشمعل قال سمعت
عن النبي ﷺ قال لا تساب وأنت صائم وإن سبك إنسان فقل إني صائم
Its isnad is sahih. Ajlan is fine as a narrator, and it is corroborated by the hadith "Fasting is a shield... every deed of Ibn Adam is for him," found in Bukhari 2/670, numbers 1795 and 1805, with our wording.